Solution of ultra-high molecular weight polyethylene

ABSTRACT

Solution of UHMWPE in an organic solvent having a total PE concentration of at least 15% by weight and in which some of the PE present is crosslinked.

The invention relates to a solution of ultra-high molecular weightpolyethylene in an organic solvent.

From "Ullmanns Encyclopedia of Industrial Chemistry", Vol. A10, (1987),page 522, paragraph 2.1.4 it is known to process polymers which cannotbe processed as a melt, for example by spinning or extruding, in theform of a solution of the polymer in a suitable solvent. In particular,this applies in the case of polymers having a high molecular weight. Inthe said reference it is also pointed out that the concentration of thepolymer in the solution must not be too low, since this lowers theproduction and increases the costs of solvent recovery. It is known thatthe viscosity of solutions of ultra-high molecular weight polymersincreases with the concentration for a constant molecular weight of thepolymer and with the molecular weight for a constant concentration. Adisadvantage of these solutions is that they already have such a highviscosity at a low concentration that the solutions frequently cannot beprocessed, or can be processed only with difficulty, using theconventional equipment. In particular in the case of the processing ofultra-high molecular weight polyethylene, a very well known and widelyused example of an ultra-high molecular weight polymer which hereinafteris indicated by UHMWPE, the concentration which is still permissiblewith regard to this restriction is frequently in conflict with economicoperation because of the large amount of solvent to be removed andfurther processed, and the relatively low yield of processed UHMWPE.

The aim of the invention is to provide a solution of UHMWPE which doesnot have this disadvantage or has this disadvantage to a considerablylesser extent.

This aim is achieved according to the invention in that the polyethyleneconcentration is at least 15% by weight and the solution contains acrosslinked ultra-high molecular weight polyethylene.

When mention is made in this description of a solution, this is alsounderstood to be, in addition to a composition consisting ofnon-crosslinked polyethylene, which is molecularly distributed in asolvent for polyethylene, a homogeneous composition of this type inwhich only crosslinked polyethylene is present or in which crosslinkedpolyethylene and uncrosslinked polyethylene occur alongside one another.

The solution according to the invention is found to possess anappreciably more favourable relationship between the total polyethyleneconcentration and the viscosity than the known solution. Consequently,more concentrated solutions than were possible hitherto can be processedusing conventional equipment or, stating the advantage of the solutionaccording to the invention in another way, the processing of solutionshaving a specific UHMWPE concentration places lesser demands on theprocessing equipment. In practice, what is achieved by this means isthat, as a result of the higher permissible concentration, for a givenflow rate in the processing equipment, for example an extruder, per unittime a greater amount of the polymer is converted into the desiredproduct and a smaller amount of solvent has to be processed. This makesa considerably more economic process possible.

A further advantage of the solution according to the invention is that,for a chosen concentration, the service life of the equipment used forprocessing can be appreciably prolonged as a result of the now loweroperating pressure, which is built up during spinning or extruding ofthe solution.

It is known from U.S. Pat. No. 2,908,659 to add specially preparedcrosslinked particles called "d-microgels" to a solution of a polymer inorder to introduce certain properties into products produced with theaid of said solution. Of such properties only dyeability is explicitlymentioned. The low-concentration solutions of UHMWPE or other highmolecular weight polymers are not mentioned in the patent. The size ofthe crosslinked particles is emphatically restricted to at most 3 μm andthe amount of crosslinked particles to be added is emphaticallyrestricted to at most the concentration associated with a gel pointdefined in this patent and measured at the processing temperature. Thisgel point is located at that concentration of crosslinked particles in asolution at which a sharp increase in the viscosity of the solutionoccurs and the said patent advises against adding higher concentrationsof crosslinked particles than those associated with the gel point. Theconcentration associated with the gel point will depend on the molecularweight of the polymer. Now it has been found that for a solution ofcrosslinked UHMWPE (HB 312 CM from Himont) with a weight-averagemolecular weight of 2.6×10⁶ kg/mol in Decalin this gel point lies at aconcentration of approximately 5% by weight measured at 140° C. Theconcentrations of crosslinked polyethylene in the solution according tothe present invention range from 3 to 100% and thus in the majority ofcases lie far above the gel point. The particle size in which UHMWPE andthus also of crosslinked UHMWPE is commercially available is at least 50μm. Since crosslinking normally does not affect the particle size thislimitation is also valid for crosslinked UHMWPE. This UHMWPE incrosslinked form can be used in the solution according to the inventionwithout any problems. Thus, in respect of the two points indicated ascritical in U.S. Pat. No. 2,908,659, that is to say particle size andconcentration of the crosslinked polymer to be used, said patentdissuades those skilled in the art from the solution according to theinvention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the measured values associated with solutions having thesame percentage of added crosslinked UHMWPE for different totalconcentrations of polyethylene.

The solution according to the invention contains UHMWPE in aconcentration of at least 15% by weight. UHMWPE is here understood to belinear polyethylene with less than 1 side chain per 100 carbon atoms andpreferably with less than 1 side chain per 300 carbon atoms and apolyethylene of this type which can also contain minor amounts,preferably less than 5 mol-%, of one or more other alkenes copolymerisedtherewith, such as propylene, butene, pentene, hexene, 4-methyl-pentene,octene, and the like, which polyethylene or copolymer of ethylene has aweight-average molecular weight of at least 0.5×10⁶ kg/kmol. Thepolyethylene can further contain minor amounts, preferably at most 25%by weight, of one or more other polymers, in particular a 1-alkenepolymer, such as polypropylene, polybutene or a copolymer of propylenewith a minor amount of ethylene. UHMWPE of this type can be produced,for example, with the aid of a Ziegler process or a Phillips processusing suitable catalysts and under known polymerisation conditions. Theweight-average molecular weight of UHMWPE is determined using themethods known for this purpose, such as gel permeation chromatographyand light scattering, or is calculated from the intrinsic viscosity (IV)determined in Decalin at 135° C. A weight-average molecular weight of0.5×10⁶ kg/kmol corresponds to an IV in Decalin at 135° C. of 5.1 dl/gin accordance with the empirical relationship

    M.sub.w =5.37×10.sup.4 [IV].sup.1.37.

The organic solvents used are the known solvents for polyolefins, suchas aliphatic, cycloaliphatic and aromatic hydrocarbons, such as toluene,xylene, Tetralin, Decalin, C₆ -C₁₂ -alkanes or petroleum fractions, butalso halogenated hydrocarbons, for example trichlorobenzene and otherknown solvents, can be applied. In connection with the removal of thesolvent, solvents are preferably used for which the boiling point underatmospheric pressure is below 210° C., which is the case for virtuallyall of the abovementioned solvents.

The way in which the solution is prepared does not constitute part ofthe invention and this preparation can take place using the techniquesknown for this purpose, such as, for example, mixing, kneading orstirring, in the known installations suitable for this purpose, or usingmethods known for the preparation of highly concentrated solutions.

In the case of a total UHMWPE concentration in the solution of below 15%by weight, the fact that some or even all of the UHMWPE is crosslinkedis found to have only a slight effect on the processibility of thesolution, as reflected in, for example, the pressure generated in theextruder during the extrusion of the solution. In the case of totalUHMWPE concentrations in the solution of more than 15% by weight, theadvantageous effect of the presence of crosslinked polyethylene (PE) inthe solution is found to be clearly expressed in the form of, forexample, a lower pressure built up during spinning or extrusion of thesolution, and in particular this applies in the case of total UHMWPEconcentrations in the solution of more than 20% by weight. Moreover, theeffect is found to increase with the percentage of crosslinked UHMWPErelative to the total amount of PE in the solution. A substantial effectis found to occur in the case of percentages of crosslinked polyethyleneof from 20%. Preferably, therefore, the amount of crosslinkedpolyethylene in the solution is 20 to 100 % and preferably 30 to 100%,calculated relative to the total amount of polyethylene.

The crosslinking of the UHMWPE can take place using methods known perse, for example, by heating, if desired in the presence of substanceswhich can effect crosslinking, for example, peroxides or silanes in thepresence of water, by electron irradiation or by a combination of thesemeasures. Preferably, the crosslinking is effected by electronirradiating of the UHMWPE to be crosslinked in powder form. This methodfor crosslinking has the advantage that no foreign substances areintroduced into the polyethylene during crosslinking, while the powderform of the crosslinked polyethylene obtained is advantageous inbringing the crosslinked polyethylene homogeneously into solution.Preferably, the crosslinking of the UHMWPE takes place at a temperaturewhich is below the melting point of the polyethylene and morepreferentially at a temperature below 80° C. Electron irradiation of thepolyethylene to be crosslinked at room temperature has been found to bea suitable method for obtaining crosslinked polyethylene suitable foruse in the solution according to the invention. If the crosslinkingtakes place at a temperature above the melting point of thepolyethylene, the powder particles melt to form larger conglomerateswhich are more difficult to bring homogeneously into solution.

Despite the increase in the total polyethylene concentration, theaddition of crosslinked polyethylene to the solution of non-crosslinkedpolyethylene is found to influence the processibility of the solutionthrough a spinning pump or extruder to only a small degree. Inparticular, this applies in the case of the pressure built up during thespinning or extrusion of the solution from a spinning or extrusionorifice, which pressure as a rule constitutes a limiting factor whenprocessing solutions having these total concentrations using processesknown per se. Consequently, a higher total UHMWPE concentration in thesolution can be chosen than in the case where only uncrosslinked UHMWPEis present in the solution.

The intrinsic viscosity of the UHMWPE, from which the crosslinkedpolyethylene has been produced, and of the non-crosslinked UHMWPE can bechosen either identical to one another or different from one another. Ifthe uncrosslinked and the crosslinked UHMWPE differ in this respect,products which possess a combination of the properties of the individualcomponents can be produced by processing the solution according to theinvention. If the uncrosslinked polyethylene and the starting materialfor the crosslinked polyethylene have the same IV, articles producedfrom the solution according to the invention are found to possessvirtually the same advantageous properties as articles produced from asolution of only uncrosslinked polyethylene having the same IV. Thisapplies in particular with regard to the chemical resistance, thestretchability, the creep, the stiffness in flexure, the abrasionresistance and the adhesion properties, in respect of both self-adhesionand adhesion to elastomers. The Vicat softening temperature of thesearticles is found to be even higher than that of articles produced froma solution in which only the non-crosslinked PE is present.

The solution according to the invention can be used for processing toarticles in cases where solutions of uncrosslinked PE can also be used.Examples are the processes known per se for the processing of suchsolutions to give, for example, filaments, tapes and films, by, forexample, extruding and spinning a solution, followed by furtherprocessing, for example, removal of the solvent, subjection to atemperature treatment and/or stretching.

The invention is illustrated with the aid of the following examples,without, however, being restricted thereto. The quantities given in theexamples are determined as described below.

The Vicat softening temperature is determined in accordance with ASTMstandard D1525-76 using a needle pressure of 5 kg.

The stretchability is determined by manual stretching on a hot plate.

The adhesive strength is determined using the 180° peeling testaccording to DIN 53530.

The tensile strength and the modulus of elasticity are determined fromthe stress/strain curve measured at room temperature using a Zwick 1435tensile tester on samples having a clamped length of 50 mm at drawingspeed of 50 mm/min.

Example I

Solutions are prepared, in Decalin, of polyethylene (HB 312 CM fromHimont) having an IV of 17 dl/g, corresponding to a weight-averagemolecular weight of 2.6×10⁶ kg/kmol, with different total concentrationsof crosslinked and uncrosslinked polyethylene together and for eachtotal concentration with different percentages of uncrosslinkedpolyethylene relative to the total amount of PE in the solution. Theradiation dose during crosslinking is 4-6 MRad. The solutions areprepared with the aid of a twin-screw extruder, to which thepolyethylene and the Decalin are fed in the form of a suspension. Thepressure which is built up during the extrusion of the solution at 140°C. through a slit having a width of 250 mm and a height of 5 mm ismeasured. The measured values associated with solutions having the samepercentage of added crosslinked UHMWPE for different totalconcentrations of polyethylene are located on the curves shown inFIG. 1. In FIG. 1 the total UHMWPE concentration in the solution isplotted, in percent by weight relative to the total amount ofpolyethylene and solvent together, along the horizontal axis and theextruder pressure built up, in bar, is plotted along the vertical axis.The percentage of crosslinked polyethylene relative to the total amountof PE for each curve can be read off from Table 1. It is pointed outthat from a total UHMWPE concentration of 15% by weight the influence ofthe presence of the crosslinked polyethylene with respect to the totalamount of PE becomes clearly discernible, in particular in the case ofcrosslinked polyethylene contents of 20% and above.

                  TABLE 1                                                         ______________________________________                                                    Percentage of                                                     Curve       crosslinked PE                                                    ______________________________________                                        A            0                                                                B           20                                                                C           30                                                                D           50                                                                E           57                                                                F           71                                                                G           86                                                                H           100                                                               ______________________________________                                    

Example II

The Vicat softening temperature is determined on a plate-shaped articleproduced by extrusion in accordance with Example I from a solution ofUHMWPE in Decalin followed by cooling of the article and removal of thesolvent therefrom by evaporation. The results are given in Table 2.

                  TABLE 2                                                         ______________________________________                                        Concentration          Vicat value                                            of UHMWPE in Decalin   in °C.                                          ______________________________________                                        15% uncrosslinked      80.2 ± 2.8                                          20% uncrosslinked      77.5 ± 1.3                                          15% uncrosslinked + 20% crosslinked                                                                  88.4 ± 3.8                                          ______________________________________                                    

The Vicat softening temperature is found to be higher when crosslinkedUHMWPE is present.

Example III

The stretchability of samples of plate-shaped articles, produced as inExample II, is determined. The results for various concentrations ofUHMWPE in the solution used are given in Table 3.

                  TABLE 3                                                         ______________________________________                                                    Total concen-                                                     Total concentra-                                                                          tration of                                                        tion of uncross-                                                                          crosslinked                                                                              Stretching Degree of                                   linked UHMWPE                                                                             UHMWPE     temperature                                                                              stretching                                  [%]         [%]        [°C.]                                                                             [-]                                         ______________________________________                                         20*         0         120        15x                                         20           5         120        15x                                         20          10         120        16x                                         20          20         120        15x                                         ______________________________________                                         *For comparison                                                          

The degree of stretching is found not to decrease when crosslinkedUHMWPE is added.

Example V

Using the procedure of Example II, plates are produced from solutionscontaining various concentrations of crosslinked and non-crosslinkedUHMWPE. Each two plates, produced from the same solution, are thenpressed against one another under a pressure of 6 bar at a temperatureof 140° C. for 6 minutes. The average strength of the mutual adhesion ofthe plates is given in Table 4.

                  TABLE 4                                                         ______________________________________                                        Total         Total                                                           concentration concentration                                                   of uncrosslinked                                                                            of crosslinked                                                                           Average adhesive                                     UHMWPE        UHMWPE     power                                                [%]           [%]        [N/cm]                                               ______________________________________                                        0             20         202                                                  0             30         173                                                  5             30         180                                                  15            20         162                                                  20*            0         160                                                  30*            0         174                                                  ______________________________________                                         *For comparison                                                          

Example VI

The modulus E and tensile strength a of plates produced in accordancewith Example II is determined before and after pressing at 160° C. andat 200° C. The results are given in Table 5.

                                      TABLE 5                                     __________________________________________________________________________    Total    Total                                                                concentration                                                                          concentration    Pressed Pressed                                     of uncross-                                                                            of crosslinked                                                                       Untreated 160° C.                                                                        200° C.                              linked UHMWPE                                                                          UHMWPE σ                                                                            E    σ                                                                           E   σ                                                                           E                                       [%]      [%]    [MPa]                                                                              [MPa]                                                                              [MPa]                                                                             [MPa]                                                                             [MPa]                                                                             [MPa[                                   __________________________________________________________________________     15*      0     27   1350 19  300 19  420                                     15       20     28   1160     550 23  580                                      0       20     26   1290 19  390                                             __________________________________________________________________________     *For comparison                                                          

The presence of the crosslinked UHMWPE does not adversely influence thetensile strength and modulus.

We claim:
 1. A composition containing ultra-high molecular weightpolyethylene and at least one organic solvent wherein the concentrationof said ultra-high molecular weight polyethylene is at least 15% byweight, and said ultra-high molecular weight polyethylene comprisescrosslinked ultra-high molecular weight polyethylene and non-crosslinkedultra-high molecular weight polyethylene wherein said non-crosslinkedultra-high molecular weight polyethylene is in solution in said organicsolvent and said crosslinked ultra-high molecular weight polyethylene isdispersed in said solution, said ultra-high molecular weightpolyethylene having a weight-average molecular weight of at least0.5×10⁶ kg/kmol which corresponds to an intrinsic viscosity (IV) inDecalin at 135° C. of 5.1 dl/g in accordance with the empiricalrelationship

    M.sub.w =5.37×10.sup.4 (IV).sup.1.37.


2. A composition according to claim 1, wherein said organic solvent istoluene, xylene, Tetralin, Decalin, a C₆ -C₁₂ -alkane, a petroleumfraction, or trichlorobenzene.
 3. A composition according to claim 1,wherein the concentration of the crosslinked ultra-high molecularpolyethylene is at least 20% by weight relative to the total amount ofultra-high molecular weight polyethylene.
 4. A composition according toclaim 1, wherein said non-crosslinked ultra-high molecular weightpolyethylene is present in an amount of at least 15 weight % and saidcrosslinked ultra-high molecular weight polyethylene is present in anamount of at least 20 wt. % relative to the total amount of ultra-highmolecular weight polyethylene.
 5. A composition according to claim 1,wherein the intrinsic viscosity of the non-crosslinked ultra-highmolecular weight polyethylene is different from the intrinsic viscosityof the crosslinked ultra-high molecular weight polyethylene.
 6. Acomposition according to claim 1, wherein the intrinsic viscosity of thenon-crosslinked ultra-high molecular weight polyethylene is the same asthe intrinsic viscosity of the crosslinked ultra-high molecular weightpolyethylene.
 7. A composition according to claim 1, wherein saidcrosslinked ultra-high molecular weight polyethylene was crosslinked ata temperature below 80° C.
 8. A composition according to claim 1,wherein said crosslinked ultra-high molecular weight polyethylene wasobtained by irradiating a non-crosslinked ultra-high molecular weightpolyethylene at room temperature.
 9. A composition according to claim 8,wherein said crosslinked ultra-high molecular weight polyethylene wasobtained by subjecting a powder consisting of non-crosslinked ultra-highmolecular weight polyethylene to electron irradiation.
 10. A compositionaccording to claim 1, wherein the ultra-high molecular weightpolyethylene has a weight-average molecular weight of 2.6×10⁶ kg/kmol.11. A process for preparing an article from a composition according toclaim 1, which process comprises extruding or spinning said compositionto obtain an article, and removing the organic solvents from thearticle.
 12. A process according to claim 11, wherein a compositionaccording to claim 9 is extruded to obtain an extruded article and thesolvent is removed therefrom.
 13. A process according to claim 11,further comprising stretching the extruded article before or afterremoving the solvent.
 14. A process according to claim 11, wherein acomposition according to claim 9 is spun to obtain a spun article andthe solvent is removed therefrom.
 15. A process according to claim 14,further comprising stretching the spun article before or after removingthe solvent.
 16. A composition containing an ultra-high molecular weightethylene (co)polymer and at least one organic solvent, wherein saidultra-high molecular weight ethylene (co)polymer contains less than 5mole percent of at least one alkene selected from the group consistingof propylene, butene, pentene, hexene, 4-methyl-petene and octene, saidultra-high molecular weight ethylene (co)polymer having a weight-averagemolecular weight of at least 0.5×10⁶ kg/kmol which corresponds to anintrinsic viscosity (IV) in Decalin at 135° C. of 5.1 dl/g in accordancewith the empirical relationship

    M.sub.w =5.37×10.sup.4 [IV].sup.1.37,

wherein the concentration of the ultra-high molecular weight ethylene(co)polymer is at least 15% by weight, and said ultra-high molecularweight ethylene (co)polymer consists of crosslinked ultra-high molecularweight ethylene (co)polymer and non-crosslinked ultra-high molecularweight ethylene (co)polymer wherein said non-crosslinked ultra-highmolecular weight ethylene (co)polymer is in solution in said organicsolvent and said crosslinked ultra-high molecular weight ethylene(co)polymer is dispersed in said solution.
 17. A composition containingan ultra-high molecular weight ethylene (co)polymer and at least oneorganic solvent, wherein said ultra-high molecular weight ethylene(co)polymer contains less than 5 mole percent of at least one alkeneselected from the group consisting of propylene, butene, pentene,hexene, 4-methyl-petene and octene, said ultra-high molecular weightethylene (co)polymer having a weight-average molecular weight of atleast 0.5×10⁶ kg/kmol which corresponds to an intrinsic viscosity (IV)in Decalin at 135° C. of 5.1 dl/g in accordance with the empiricalrelationship

    M.sub.w =5.37×10.sup.4 [IV].sup.1.37,

wherein the concentration of the ultra-high molecular weight ethylene(co)polymer is at least 15% by weight, and said ultra-high molecularweight ethylene (co)polymer consists of crosslinked ultra-high molecularweight polyethylene in an amount of 20%-100% by weight and 80% to 0% byweight non-crosslinked ultra-high molecular weight polyethylene, whereinsaid non-crosslinked ultra-high molecular weight ethylene (co)polymer isin solution in said organic solvent and said crosslinked ultra-highmolecular weight ethylene (co)polymer is dispersed in said solution, andwherein said % by weight being relative to the total amount ofultra-high molecular weight ethylene (co)polymer in solution.
 18. Anultra-high molecular weight polyolefin composition comprising at leastone organic solvent and an ultra high molecular weight polyolefinselected from the group consisting of ultra-high molecular weightethylene homopolymer, ultra-high molecular weight ethylene copolymer,and mixtures thereof, said ultra-high molecular weight ethylenecopolymer and said ethylene homopolymer each have a weight-averagemolecular weight of at least 0.5×10⁶ kg/kmol which corresponds to anintrinsic viscosity (IV) in Decalin at 135° C. of 5.1 dl/g in accordancewith the empirical relationship

    M.sub.w =5.37×10.sup.4 [IV].sup.1.37,

the concentration of said polyolefin is at least 15% by weight, saidethylene copolymer is selected from the group consisting of crosslinkedultra-high molecular weight ethylene copolymer and non-crosslinkedultra-high molecular weight ethylene copolymer, said ultra-highmolecular weight ethylene homopolymer is selected from the groupconsisting of crosslinked ultra-high molecular weight ethylenehomopolymer and non-crosslinked ultra-high molecular weight ethylenehomopolymer, said composition contains at least one crosslinked memberselected from the group consisting of crosslinked ultra-high molecularweight ethylene copolymer and crosslinked ultra-high molecular weightethylene homopolymer, said composition contains at least onenon-crosslinked member selected from the group consisting ofnon-crosslinked ultra-high molecular weight ethylene copolymer andnon-crosslinked ultra-high molecular weight ethylene homopolymer, andsaid non-crosslinked member is in solution in said organic solvent andsaid crosslinked member is dispersed in said solution.
 19. A compositionaccording to claim 18, wherein said ultra-high molecular weight ethylene(co)polymer contains less than 5 mole percent of at least one alkeneselected from the group consisting of propylene, butene, pentene,hexene, 4-methyl-pentene and octene.